Contractile protein synthesis is being studied in cultured embryonic quail and chick skeletal muscle cells. We have shown that the synthesis of 12 contractile proteins (moysins tropomyosins, troponins, and actin) is activated coordinately when myoblasts fuse to form muscle fibers. Analyses of mRNA synthesis, stability and accumulation in differentiating muscle indicate that this coordinate activation is controlled at the level of transcription of the contractile protein mRNAs. We are currently using recombinant DNA techniques to clone sequences complementary to the individual contractile protein mRNAs for studies of the transcription and processing of nuclear precursors to the contractile protein mRNAs and the sequence and chromosomal organization of this group of coordinately regulated genes.